The inhibition of nitrite oxidizing bacteria (NOB) is a precondition for the implementation of short-cut biological nitrogen removal (ScBNR) processes such as nitritation-denitritation (Ciudad et al., 2005; Gee and Kim, 2004; Ju et al., 2007; Yoo et al., 1999; Yu et al., 2000; Zeng et al., 2008), nitrite-shunt and partial nitritation-anammox (Fux et al., 2002; Hippen et al., 1997; van Dongen et al., 2001; Wett, 2006; Wett, 2007; Wett et al., 2010), and deammonification. Successful repression of nitrite oxidation by controlling NOB saves 25% oxygen and 40% organic carbon compared to conventional nitrification-denitrification (Turk and Mavinic, 1986; Abeling and Seyfried, 1992). In deammonification processes, the control of NOB results in added benefits in further reductions in aeration energy required, and reduced costs of electron donor and solids handling. FIG. 1, FIG. 2 and FIG. 3 show flowcharts for nitrogen removal through conventional nitrification/denitrification, nitritation/denitritation and deammonification (partial nitritation+anaerobic ammonia oxidation), respectively.
In view of high cost of biological nutrient removal (BNR) to meet increasingly stringent effluent standards, ScBNR through repression of NOB is a topic of interest. Efforts to understand NOB repression have been discussed in many publications, including those that are more specific to the use of high temperature (Helling a et al., 1998), high levels of free ammonia inhibition, or dissolved oxygen (DO) concentration (Blackburne et al., 2008) and transient anoxia (Kornaros and Dokianakis, 2010). Particularly, all of these conditions are used in part or as a whole, in various approaches, with success in controlling NOB in systems treating ‘high strength’ (high free ammonia) waste streams, such as anaerobic digester dewatering liquor (also usually at high temperature) and landfill leachate. Control of NOB repression in low strength waste streams such as domestic wastewater remains a challenge and is the subject of this invention. The status quo associated with the above features and the controls developed for this invention are described below.
Temperature and Ammonia:
Both temperature and free ammonia are features believed to provide an advantage to ammonia oxidizing bacteria (AOB) over NOB. Free ammonia (FA) inhibition of NOB has been well-documented in literature ever since it was considered by Anthonisen et al. (1976). However, knowledge of controlling FA inhibition to obtain stable nitritation is more limited since NOB adaptation has been reported (Turk and Mavinic, 1989; Wong-Chong and Loehr, 1978). Further, high temperature is known to favor growth of AOB over NOB (Kim et al., 2008).
The increased activity of AOB compared to NOB at higher temperature, greater disassociation of total ammonia to free ammonia and resulting NOB inhibition at higher temperatures, combined with low DO operation (often conducted using intermittent aeration and with managed aerobic solids retention time (SRT)), results in enrichment of AOB and selective wash out of NOB. These approaches are variously described (EP 0826639 A1, EP 0872451 B1, US 2010/0233777 A1, U.S. Pat. No. 7,846,334 B2, U.S. Pat. No. 6,485,646 B1, WO 2012/052443 A1) to control NOB in ‘high strength’ wastewater. These methods either use suspended growth (WO 2006/129132 A1), attached growth on the support media (US 2011/0253625 A1, EP 0931768 B1) or granular sludge (Wett, 2007; U.S. Pat. No. 7,846,334 B2) to accomplish ScBNR.
In spite of being effective, the role of elevated temperature to increase activity of AOB and for the control of NOB growth is not feasible in low strength mainstream processes operating under wide range of temperatures. Consequently, NOB control in low strength wastewater remains intractable and requires careful manipulation of factors other than temperature or free ammonia.
Dissolved Oxygen:
Dissolved oxygen (DO) can play a significant role in control of NOB in low strength wastewater. Sustained nitritation with the use of low DO concentration has been observed in a variety of reactor configurations (Sliekers et al., 2005; Wyffels et al., 2004; Blackburne et al., 2008). Although, all of these reports lack account of underlying mechanisms, they resort to a hypothesis of higher oxygen affinity of AOB compared to the NOB (Hanaki et al., 1990; Laanbroek and Gerards, 1993; Bernet et al., 2001) as an explanation for the observed phenomenon (Yoo et al., 1999; Peng et al., 2007; Lemaire et al., 2008; Gao et al., 2009; Zeng et al., 2009). Although the hypothesis that low-DO operation favors AOB versus NOB is very widespread (see review of oxygen half-saturation parameters in Sin et al., 2008) some research results point in the opposite direction (Daebel et al., 2007; Manser et al., 2005) and also inventors' data (see FIG. 4 and FIG. 5) indicates stronger adaptation to low DO-concentration for NOB compared to AOB.
Bioaugmentation:
The transfer of nitrifying biomass from a high strength reactor to the low strength mainstream reactor, such that the SRT required to perform nitrification is decreased in the mainstream process has been reported before (U.S. Pat. No. 7,404,897 B2, U.S. Pat. No. 6,602,417 B1). This bioaugmentation can occur from a separate sidestream reactor or a reactor co-joined with the mainstream reactor (Parker and Warmer, 2007). There is also existing prior art related to the physical separation of a more dense biomass fraction containing predominantly anammox organisms and recycling this heavier fraction by the use of the hydrocyclone in order to enrich this very slowly growing biomass (EP 216352481, US 2011/0198284 A1).
Transient Anoxia:
The use of transient anoxia has been a common approach to achieve NOB repression (Li et al., 2012; Ling, 2009; Pollice et al., 2002; Rosenwinkel et al., 2005; Zekker et al., 2012; U.S. Pat. No. 7,846,334 B2; EP 0872451 B1; WO 2006/129132 A1). Transient anoxia allows for a measured approach to control the aerobic SRT as well as to introduce a lag-time for NOB to transition from the anoxic to aerobic environment. Kornaros and Dokianakis (2010) showed delay in NOB recovery and NOB lag adaptation in aerobic conditions following transient anoxia, thus confirming the observations of the usefulness of transient anoxia by many others (Allenman and Irvine, 1980; Katsogiannis et al., 2003; Sedlak, 1991; Silverstein and Schroeder, 1983; Yang and Yang, 2011; Yoo et al., 1999). Although transient anoxia has been used successfully to control NOB in ‘high strength’ wastes (Wett, 2007; U.S. Pat. No. 7,846,334 B2) and the ability to use it in low strength wastes has been suggested (Peng et al., 2004), the ability to control the features associated with transient anoxia remains an enigma until this invention.
To summarize, there exists a need for clear control strategies that exploit the underlying mechanisms of maintaining high AOB oxidation rates while achieving NOB repression with 1) ammonia and the use of ammonia setpoints, 2) operational DO and the proper use of DO setpoints, 3) bioaugmentation of a lighter flocculant AOB fraction, and 4) proper implementation of transient anoxia within a wide range of apparatus (reactor configurations) and operating conditions.